Broadcast receivers in android

Broadcast Receiver

 Class Overview

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Base class for code that will receive intents sent by sendBroadcast(). 

If you don't need to send broadcasts across applications, consider using this class with LocalBroadcastManager instead of the more general facilities described below. This will give you a much more efficient implementation (no cross-process communication needed) and allow you to avoid thinking about any security issues related to other applications being able to receive or send your broadcasts.

You can either dynamically register an instance of this class with Context.registerReceiver() or statically publish an implementation through the <receiver> tag in your AndroidManifest.xml.
Note:    If registering a receiver in your Activity.onResume() implementation, you should unregister it in Activity.onPause(). (You won't receive intents when paused, and this will cut down on unnecessary system overhead). Do not unregister in Activity.onSaveInstanceState(), because this won't be called if the user moves back in the history stack.
There are two major classes of broadcasts that can be received:

• Normal broadcasts (sent with Context.sendBroadcast) are completely asynchronous. All receivers of the broadcast are run in an undefined order, often at the same time. This is more efficient, but means that receivers cannot use the result or abort APIs included here.
• Ordered broadcasts (sent with Context.sendOrderedBroadcast) are delivered to one receiver at a time. As each receiver executes in turn, it can propagate a result to the next receiver, or it can completely abort the broadcast so that it won't be passed to other receivers. The order receivers run in can be controlled with the android:priority attribute of the matching intent-filter; receivers with the same priority will be run in an arbitrary order.

Even in the case of normal broadcasts, the system may in some situations revert to delivering the broadcast one receiver at a time. In particular, for receivers that may require the creation of a process, only one will be run at a time to avoid overloading the system with new processes. In this situation, however, the non-ordered semantics hold: these receivers still cannot return results or abort their broadcast.
Note that, although the Intent class is used for sending and receiving these broadcasts, the Intent broadcast mechanism here is completely separate from Intents that are used to start Activities with Context.startActivity(). There is no way for a BroadcastReceiver to see or capture Intents used with startActivity(); likewise, when you broadcast an Intent, you will never find or start an Activity. These two operations are semantically very different: starting an Activity with an Intent is a foreground operation that modifies what the user is currently interacting with; broadcasting an Intent is a background operation that the user is not normally aware of.
The BroadcastReceiver class (when launched as a component through a manifest's <receiver> tag) is an important part of an application's overall lifecycle.

Receiver Lifecycle

A BroadcastReceiver object is only valid for the duration of the call to onReceive(Context, Intent). Once your code returns from this function, the system considers the object to be finished and no longer active.
This has important repercussions to what you can do in an onReceive(Context, Intent) implementation: anything that requires asynchronous operation is not available, because you will need to return from the function to handle the asynchronous operation, but at that point the BroadcastReceiver is no longer active and thus the system is free to kill its process before the asynchronous operation completes.
In particular, you may not show a dialog or bind to a service from within a BroadcastReceiver. For the former, you should instead use the NotificationManager API. For the latter, you can use Context.startService() to send a command to the service.

For more details check thin link

Usage

Example :

In you MainClass :

     MyBroadcastReceiver mReceiver;                                //declare broadcast receiver object
    private boolean mIsReceiverRegistered=false;              //boolean value which will check weather receiver is null or not



    private class MyBroadcastReceiver extends BroadcastReceiver
    {

        @Override
        public void onReceive(Context context, Intent intent)
        {

//any working you want to perform in your parent class goes here.

        }
    }



@Override
    protected void onResume()
    {
        super.onResume();

        if (!mIsReceiverRegistered)
        {
            if (mReceiver == null)
                mReceiver = new MyBroadcastReceiver();         //register your receiver
            registerReceiver(mReceiver, new IntentFilter("com.myreceiver.example"));
            mIsReceiverRegistered = true;
        }
    }

    @Override
    protected void onPause()
    {
        super.onPause();
        if (mIsReceiverRegistered)
        {
            unregisterReceiver(mReceiver);             //unregister your receiver
            mReceiver = null;
            mIsReceiverRegistered = false;
        }
    }


Now,from any where you can call this broadcast receiver using intent .,it will perform anything which you want (only if your activity is open)

Like you can use this receiver to update you activity on GCM notification receive ,just call this receiver using intent. OR change UI in main thread from some other thread.

                Intent intent = new Intent("com.myreceiver.example");
                context.sendBroadcast(intent);